1. Field of the Invention
The invention is generally directed to a spectrum analyzer which has a dedicated processor for each focal plane pixel sensor in order to perform an immediate pixel-based calculation of the power spectral density of the illumination reflected from a target. In this manner, a spectrum analyzer is provided with significantly improved signal-to-noise ratio.
2. Description of the Related Art
In general, one of the most important factors that allows any electro optical system to function as designed is the signal-to-noise (SNR) ratio obtained for a given situation. In general, a larger SNR results in better, or at least more reliable operation. A poor, or low SNR causes faulty and unreliable or poor performance. In a typical electro-optical sensor, a sensor is positioned in the instrument in the normal manner it was designed for, both electrically and optically. This usually consists of a suitable collective aperture to collect the light, an optical instrument to focus the light in the sensor, and suitable electronics to record the photoelectrons generated by the sensor. In most common sensors, the output signal results from the measurement of photoelectrons generated from interaction of the incoming laser light source with the detector substrate. For visible light sensors, for example, photons produce electrons which are generated from hole pairs produced in an NPN junction. This change in balance generates a current which is measured as a resistor change (photoconductive mode), or as a voltage change (photoconductive mode). We will refer to this type of sensor as a “square law” detector, which gets its name from the fact that the light intensity is proportional to the square of the electrical field of the incident light wave. In this case, either the current or voltage output by the sensor is proportional to the amplitude of the intensity of the light received by the sensor.
The problem with this sort of typical device is that any photons received will be recorded as photoelectrons over a wavelength operating capability of the device. Sunlight, moonlight, starlight, scattered laser light from aerosols or clouds, urban street lights or car lights, and battlefield explosions or other stray light sources, all contribute to the background “noise” recorded by the device. With respect to these types of devices, “photon noise” is usually defined as any photoelectrons generated, which are not part of the target's signal but which contribute to the final current or voltage output by the sensor.
In addition to other photon sources generating the noise signal, any electrical disturbances which contribute to the current or voltage output by the sensor, but which are not part of the target signal, cause additional noise. Entire companies, and an active industry, are employed in building suitable electronic amplifiers, filters, and circuits designed to reduce these purely “circuit noise” effects.
Accordingly, it is desirable to develop an electro-optical sensor device which can reduce or eliminate the “photon noise” from the sensor's output, while providing an accurate measurement of the target illumination detected by the sensor.